At one stage in the production of planks and boards from logs, the lumber workpieces are in the form of cants or waney planks having substantially flat and parallel top and bottom face surfaces and more or less irregular longitudinally extending side surfaces. Because each workpiece is to be converted into one or more boards having finished side surfaces that are straight, parallel and accurately perpendicular to the already-finished top and bottom surfaces, a certain amount of material must be removed in an edge trimming operation by which the side surfaces are finished.
In the interests of economy, the edge trimming cuts are so planned that the least possible amount of material will be wasted. Usually the planning for the trimming operation is accomplished as the workpiece moves to and arrives at an orienting station. For such planning, the workpiece has its narrower face surface uppermost, so that an estimate can be made of the orientation and spacing of the edging cuts that will enable the most profitable standard size board or plank to be produced from the workpiece. The planning may be done by a skilled operator, or may be done automatically by means of apparatus such as is disclosed in U.S. Pat. Nos. 3,886,372, 3,963,938 and 3,970,128. With automatic apparatus, the workpiece is passed under a scanning device that is associated with a computer, and, on the basis of the geometry of the top face surface of the workpiece, as ascertained by the scanning apparatus, the computer produces outputs which control the adjustment of orientation instrumentalities. In the absence of automatic scanning and orienting apparatus, a skilled operator orients the workpiece on the basis of his observation of the geometry of the finished top surface and his estimate of the edge finishing cuts that will result in the optimum finished product.
Whether the orientation is performed by automatic apparatus or by a skilled person, the result of the orientation process is that a relationship is established between the workpiece and the side edge trimming instrumentalities such that the desired trimming cuts can be made on it by feeding it from the orienting station through the trimming station with a purely translatory motion, such motion of course being in a horizontal direction and substantially lengthwise of the workpiece.
It will be evident that the calculation and planning which culminates in positioning of the workpiece at the orienting station will be useful only insofar as the workpiece is prevented from changing its orientation as it is fed from the orienting station through the trimming station. Heretofore, workpieces have been fed to the trimming station by means of a roller conveyor such as is disclosed in the above mentioned U.S. Pat. Nos. 3,963,938 and 3,970,128. Such a conveyor comprised a number of identical cylindrical rollers having their axes extending horizontally and transversely to the direction of feed. The rollers were rotatably driven in unison and were arranged at close enough intervals along the path of feed so that each workpiece could always rest on a plurality of rollers, to be advanced by their rotation. To prevent slippage between the rollers and the workpiece, freely rotatable hold-down rollers, mounted at intervals along the feed path, engaged against the top surface of the workpiece as it moved along.
It has been found that a roller conveyor could not be relied upon to effect a purely translatory motion of the workpiece. At the trimming station, the trimming saws or cutters exerted lateral forces on the workpiece, and the roller conveyor allowed it to be shifted out of its desired orientation in response to such forces, the shifting often continuing and becoming progressively worse as the piece advanced through the trimming station. If the top surface of the workpiece was not parallel to its bottom surface, the hold-down force exerted by the holding rollers could be concentrated along one longitudinal edge of the workpiece, and the workpiece could be laterally shifted even before it reached the trimming station. Other circumstances such as warped or crooked workpieces could also cause lateral shifting whereby the trimming was not accomplished along the calculated cutting lines.
Any displacement of the trimming cuts from their desired locations or orientations, even by a few millimeters, could result in a board or plank which was totally useless or which had to be cut down to a less profitable size, either by shortening it or by reducing its width.
At least in the case where orientation of workpieces is performed by means of automatic apparatus comprising a scanner and a computer, the problem of feeding the workpieces to the trimming station with a purely translatory substantially lengthwise motion is complicated by the necessity for moving each workpiece to the orienting station in a direction transverse to its length and bringing it up against stops at the orienting station that engage its leading side edge at two points that are spaced from one another along its length. Under control of the computer, the stops are adjustable in directions transverse to the length of the workpiece, and the positions to which they are adjusted determine the orientation of the workpiece at the orienting station. The workpiece must of course be transported to the orienting station by means of a conveyor or the like which assures engagement of the workpiece against the orienting stops but does not continue to apply substantial driving force to the workpiece after it is arrested by the orienting stops.
It will be apparent that the means employed for effecting translatory substantially lengthwise feeding movement of workpieces from the orienting station to the trimming station must bring the workpiece into such translatory motion from a complete stop without causing or permitting any change in the orientation of the workpiece, but nevertheless must not interfere with lateral movement of the workpieces into the orienting station, nor with adjusting movements of the orienting stops, nor with proper engagement of each workpiece against the orienting stops.